Most conventional techniques for digital to analog conversion require both high performance analog circuitry, such as operational amplifiers, and digital circuitry for counting, sequencing, and data storage. This has tended to result in hybrid circuits consisting of one or more bipolar analog integrated circuit chips and an MOS chip to perform the digital function. High performance and fairly complex signal processing has recently become possible on a single monolithic chip due to the fact that very accurate capacitor ratios could be obtained in MOS technology to replace cumbersome conventional techniques such as diffused resistors, complex thin film process, or ion implanted resistors. One problem with existing integratd circuits is that the area of silicon occupied by capacitors increases exponentially as a function of the number of input bits. Hence, in the prior art it has been difficult to get enough accuracy for ten bit resolution with reasonable silicon area. Also, for high speeds (say, 10.sup.7 conversions/second) the amplifiers may not be fast enough.
The present invention discloses a series of three capacitor and switch sections to convert a three bit segment of a digital word. For a ten bit digital word, for example, three 3-capacitor sections would be required with an additional capacitor section. The voltage across the output, or last, capacitor is the analog voltage in direct relation to the input digital word. Each of the three capacitor sections works in relation to switched transistors to charge and discharge, in a predetermined fashion, said capacitors, in relation to the binary level of each of the three digits in the input digital word. The circuit operates from the least significant bit to the most significant bit, and converts the input digital word to an output analog voltage.